Hydrometallurgy of Lead

NEW TECHNOLOGY FOR LEAD

Fathi Habashi Department of Mining, Metallurgical, and Materials Engineering Laval University, Quebec City, Canada e-mail: [email protected]  Lead is an ancient metal, has been produced to-date from its ores exclusively by pyrometallurgical route.  The process suffers from numerous steps, high operating cost, and excessive pollution problems.

Galena PbS Metallurgy of lead Refining of lead Refining of lead • The complex refining steps and the pollution in the neighborhood of smelters are causing much trouble to the nearby population • For example, the high content of lead in crab collected from the ocean in the vicinity of a lead smelter worries consumers • The appreciable amounts of lead in wine produced from a vineyard near a lead smelter causes concern to the industry.

• The maximum permissible limit for lead in the vicinity of a smelter is 0.05 mg Pb/m3 of air or 6 ppb, a limit that is difficult to achieve in many plants.

• In addition, lead smelters produce SO2 and this must be converted to sulfuric acid. • A nearby market for the acid must exist. otherwise SO2 will have to be emitted in the environment, which is unacceptable. • As a result, a number of smelters and refineries have been shut down.

• Both problems can only be solved by using a hydrometallurgical route to process the sulfide concentrate. • No lead fumes will be emitted in the environment and elemental sulfur could be produced, which is easy to store or ship to sulfuric acid manufacturers.

Hydrometallurgy of lead

• Extensive research has been going on since the beginning of the twentieth century to find a non- polluting process for its production and a solution for its complex refining scheme. • It has been assumed correctly that the hydrometallurgical route should be the most promising. • Pilot plants have been constructed and operated for a reasonable periods, but no process has proved to be fully satisfactory until recently.

First hydrometallurgical attempts

• O. C. Ralston US Bureau of Mines in Berkeley, California 1924 The solubility of

PbCl2 and PbSO4 in brine solutions Tainton at Bunker Hill in Kellogg, Idaho 1924

PbS concentrate O2 Roasting SO2

H2O PbSO4, impurities Leaching

Filtration Impurities CaCl2

Leaching

Filtration Gangue, CaSO4

2- PbCl4 Cl2 Absorption Aq. Electrolysis

Ca(OH)2 Refining Ag

Pb Failure of Tainton Process

• Although it was supposed to be a sulfation roasting,

yet some SO2 was formed. • In the electrowinning step chlorine was formed and was not disposed of properly beside its corrosion problems. • Lead powder obtained was not satisfactorily handled and was contaminated by silver. Bunker Hill Process in Kellogg, Idaho (1960s). High temperature aqueous oxidation (220oC) PbS Concentrate

O2 H2O

Aq. Oxidation

Filtration CuSO4, ZnSO4

PbSO4, Ag2SO4

Blast Furnace CO2, SO2 Crude Pb

Refining Ag

Pb Problems with Bunker Hill Process

• The process was thought to be more advantageous than the roasting process

because no SO2 would be evolved.

• In fact it was not, since SO2 was generated in the blast furnace gases due to the decomposition of lead sulfate.

Other processes

Galena is attacked by dilute acids generating H2S: + 2+ PbS + 2H → Pb + H2S which can be collected and converted by standard technology to elemental sulfur at 400ºC using alumina as catalyst:

H2S + ½ O2 → S + H2O

However, the toxicity of H2S and its explosive nature renders this route undesirable Galena is attacked by concentrated H2SO4 at 100ºC to form SO2 and elemental sulfur:

PbS + 2H2SO4 → PbSO4 + SO2 + S + 2H2O

The reaction is simple but offers no special advantage since SO2 is generated.

Aqueous oxidation of PbS in acid

Aqueous oxidation of PbS in acid results in the formation of elemental sulfur: + 2+ PbS + ½ O2 + 2H → Pb + S + H2O

• When H2SO4 is used, PbSO4 will be formed and when HCl is used then PbCl2 will be formed together with an appreciable amount of PbSO4 since a portion 2- of sulfide sulfur is oxidized to SO4 and other components of the concentrate will form soluble sulfates. • Solution purification can be achieved by cementation of the impurities with lead powder. Ferrous ion can then be oxidized back to ferric for recycle. Oxidation may take place by oxygen: 2+ + 3+ Fe + ½O2 + 2H → 2Fe + H2O

• Or by chlorine when FeCl3 is used: 2+ 3+ - 2Fe + Cl2(aq) → 2Fe + 2Cl

There is no advantage in using the sulfate system because of the difficulties encountered in the recovery step. When Fe3+ ion is used instead of oxygen the following reaction takes place: PbS + 2Fe3+ → Pb2+ + 2Fe2+ + S

Processing lead sulfide concentrates with formation of elemental sulfur. Chloride system

PbS concentrate Fe3+ HCl Aq. Oxidation

Filtration Soluble chlorides

Flotation S Brine Leaching

Filtration Gangue

Fe 2+ oxidation Crystallization

PbCl2 Cl2 Electolysis

Pb US Bureau of Mines Process in Reno, Nevada • In this case chlorine could be obtained from

the electrolysis of PbCl2 either in aqueous solution (complexed with NaCl) or in the molten state. • This was the basis of the processes developed by researchers at US Bureau of Mines and others. • This shows again that the chloride system is more preferable than the sulfate system, since in the latter case the sulfate ion must be disposed of. • Oxidation of Fe2+ may also be achieved in the electrolytic step whereby the evolution of chlorine is suppressed as proposed by French researchers.

Carbonate system

To avoid the formation of PbSO4 or PbCl2, aqueous oxidation of PbS was conducted by Chinese researchers in presence of ammonium carbonate at about 50°C. Lead carbonate and elemental sulfur are formed:

PbS + (NH4)2CO3 + ½O2 + H2O → PbCO3 + S + 2NH4OH

• Residence time about 6 hours and yield of sulfur is 60%. After flotation of elemental sulfur, PbCO3 was solubilized in fluorosilicic acid, the solution purified, then electrolyzed for electrowinning of lead. PbS concentrate (NH4)2CO3 O2

Aq. Oxidation

Filtration Recovery Solids

Flotation S, Ag

H2SiF6 PbCO3, gangue Dissolution

Filtration Residue containing Ag

Purification Impurities

Electrolysis

Pb NITRATE SYSTEM

• The nitrate system has the advantage that both lead and silver will go into solution and hence separation can be readily achieved.

• Using HNO3, however, has the disadvantage of generating nitric gases, which must be re-

converted to HNO3.

• The use of ferric nitrate was already proposed. the following reaction takes place:

PbS + 2Fe(NO3)3 → Pb(NO3)2 + 2Fe(NO3)2 + S • Complete dissolution of galena took place at 70°C in 0.25 M Fe(NO3)3 solution at pH 1.2- 1.4 in 100 minutes.

• While lead forms at the cathode, PbO2 forms at the anode. PbS concentrate Fe(NO3)3

Leaching

Filtration Flotation Solution

Heating Gangue S NO + NO2

Regeneration Filtration

HNO3 Fe 2O3 Solution

Dissolution Purification

Bi, Cu, Ag, Zn Electrolysis

Pb + PbO2 – After purification of the solution from copper and bismuth by cementation on lead and the separation, if necessary, of zinc by organic solvents, lead can be recovered by electrowinning at the cathode as Pb and at the anode as PbO2:

- - Pb(NO3)2 + 2e  Pb + 2NO3 - - Pb(NO3)2 + 2H2O + 2NO3  PbO2 + 4HNO3 + 2e

Overall reaction

2Pb(NO3)2 + 2H2O  Pb + PbO2 + 4HNO3

CHLORINATION

The use of gaseous chlorine has the advantage over the aqueous chloride system is the

absence of PbSO4 formation since all the sulfide sulfur is transformed to the elemental form. There have been early attempts in this direction at the beginning of the twentieth century but without success owing to the difficulties encountered in handling chlorine. About sixty years later, researchers at the US Bureau of Mines in Rolla, Missouri re-examined this technology and recommended the recovery of lead by the

electrolysis of fused PbCl2. A pilot plant was later operated at Hazen Research Center in Golden, Colorado based on such technology. Chlorine gas is fed to rotating reactor counter-current to the flow of fresh concentrate so that any sulfur monochloride formed becomes the chlorinating agent:

PbS + Cl2  PbCl2 + S

2S + Cl2  S2Cl2

PbS + S2Cl2  PbCl2 + 3S

The temperature in the reactor is155-175C.

• Lead chloride and other chlorinated compounds are then solubilized in hot brine solution. Lead chloride is then crystallized and, the anhydrous PbCl2 is fed in a fused salt cell containing 90% PbCl2 and 10% NaCl and operating at 500C. • High purity lead was obtained. • Mother liquor from crystallization step is treated with sponge iron to remove silver. A bleed stream is treated with NaOH or Na2CO3 to remove other impurities. • Work at Universal Oil Products laboratory in Des Plaines, Illinois also confirmed this technology. FLUOROSILICATE SYSTEM

• Since lead fluorosilicate is soluble in water, it was suggested by workers at US Bureau of Mines in Rolla, Missouri to leach lead sulfide concentrate in fluorosilicic acid:

• PbS + H2SiF6 + ½O2  PbSiF6 + S + H2O

• After filtration of the residue and solution purification, lead can be recovered by electrolysis. The residue contained elemental sulfur, silver, zinc, and copper. • Attempts to electrowin lead from the leach solution were not successful because of low current efficiency and undesirable cathode morphology due to impurities present. • To overcome this problem, the Bureau of Mines researchers suggested adding H2SO4 to precipitate PbSO4, transform the sulfate into carbonate, dissolving PbCO3 in H2SiF6, then electrolyzing the pure lead fluorosilicate solution. • High purity lead was obtained but the procedure suffers from the numerous steps involved and the generation of ammonium sulfate as a by-product.

PbS concentrate H SiF O 2 6 2 Leaching

Solids Filtration

Solution

Impurities Purification Flotation S

Electrolysis Silver recovery

Pb FLUOROBORATE SYSTEM

Researchers at Engitec Tehnologies in Milan, Italy found out that galena concentrates was solubilized in fluoroboric acid containing ferric fluoroborate at 80oC liberating elemental sulfur while silver, copper, bismuth, and antimony remain in the residue:

PbS + 2Fe(BF4)3 → Pb(BF4)2 + 2Fe(BF4)2 + S

• The solution is then electrolyzed in a diaphragm cell where pure compact lead is deposited at the cathode and the ferrous fluoroborate is oxidized at the anode to ferric fluoroborate for recycle:

• Pb2+ + 2e- → Pb • Fe2+ → Fe3+ + e-

PbS concentrate

HBF4 + Fe(BF4)3 Leaching

Solids Filtration Solution

Bleed for zinc recovery Electrolysis Flotation S

Silver recovery Lead • No oxygen is formed at the anode and this is a great advantage because the voltage will be lower. • Air is sparged at the anodes to prevent the

formation of PbO2. • A typical electrolyte will have the following composition in g/L: Pb2+ 50-70, Fe2+ 25-30, Fe3+ 35-30, fluoroboric acid 35- 45, total HBF4 325-335. • Temperature of electrolysis 35oC, anodic and cathodic current densities 200-250 A/m2, and graphite anodes can be used. • Cell voltage at 300 A/m2 is 2.10 volts, the current efficiency is 96%, and energy consumption is 570 kWh/tonne Pb cathode. • Lead produced is 99.99% and the overall recovery is 96.1%. Zinc is partially solubilized in the leaching step but not electrodeposited; it can be recovered from the bleed solution.

Fluoroboric acid

• Fluoroboric acid, HBF4, is prepared industrially by reacting hydrofluoric acid with boric acid:

H3BO3 + 4HF → HBF4 + 3H2O

• The reaction is exothermic and the acid is available only as a 48% solution. • Ferric fluoroborate is prepared by reacting the acid with Fe2O3. • Fluoroboric acid is more expensive than fluorosilicic acid but it has the advantage of being more stable on heating and its solutions have higher electrical conductivity. • Fluoroboric acid was used for more than fifty year by Norddeutsche Affinerie in Germany for refining lead bullion with high bismuth content. • Presently it is used in electrodeposition of lead and its alloys, especially in printed circuit board manufacture.

• Doe Run had a demonstration plant in southeast Missouri, running for more than 3 years. Now it is in stand-by. • In the same location in Missouri there is a small unit (50 kg/d) that is available for testing. • It is expected to invest more than $150 million. Doe Run, Missouri

CONCLUSIONS

• Zinc had a similar history like lead up to 1980s. It was produced exclusively by roasting the sulfide concentrate to form zinc oxide, thermal reduction of the oxide, then refining the crude metal by vacuum distillation. • During World War I, leaching of the oxide and electrowinning of zinc from the purified solution replaced the reduction and vacuum distillation steps. • In 1980, the total hydrometallurgical route, i.e., aqueous oxidation of the sulfide concentrate to get zinc sulfate solution and elemental sulfur was introduced on industrial scale. • Copper had also a similar history until at the beginning of the twenty first century when the aqueous oxidation of copper sulfide concentrates was also introduced on industrial scale by Phelps-Dodge in Arizona. • Will lead follow a similar situation? • The answer is yes.